Finished my first build this week

I

ibzprestige

Active member
Bought the Mojotone British 50 Watt Master Volume kit, recieved it last Saturday and finished mid last week. I’ve done mods before but this was my first from scratch build, It was a lot of fun. I know some of the longer wires can be cleaned up, but there’s absolutely no noise when it’s on and turned up, so I’m in no rush with cleaning that up. Now looking for some little tweaks I can do to make it a little meaner, anyone have any suggestions? It is a 1978 JMP 2204 circuit.

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Wiring diagrams:

https://www.mojotone.com/Amp_Kits/British/British_50W_1978_2204_WD.pdf
 
I like 2k7 with 0.68 bypass caps on both sides of v1. Might also try getting rid of the plate snubber, a 33k to the first grid and a 150k on the first plate. Maybe some diodes too, they’re pretty easy to install on a switch.

Check your B+ to the pre as well. Might try bumping those voltages up to 2203ish specs.
 
Chubbs said:
I like 2k7 with 0.68 bypass caps on both sides of v1. Might also try getting rid of the plate snubber, a 33k to the first grid and a 150k on the first plate. Maybe some diodes too, they’re pretty easy to install on a switch.

Check your B+ to the pre as well. Might try bumping those voltages up to 2203ish specs.
Click to expand...
Thank you, I will clip in some of those values and mess with it.

On this circuit where would I wire in the switch for clipping? I did the Headfirst Origin Jose mod with the 3 way switch, and that amp is pretty angry.
 
ibzprestige said:
Thank you, I will clip in some of those values and mess with it.

On this circuit where would I wire in the switch for clipping? I did the Headfirst Origin Jose mod with the 3 way switch, and that amp is pretty angry.
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I do it like this… basically just the diodes tied in off the 100k with a switch to ground.
 

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Chubbs said:
I do it like this… basically just the diodes tied in off the 100k with a switch to ground.
Click to expand...
Perfect, thanks!

I clipped in a 3.3k resistor and .68 cap to the stock 10k on the one side of v1, and wow, already a huge difference in gain and aggressiveness. I have a little bit of hiss now though with this set up, could be due to it not being soldered in.

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Chubbs said:
Nice, I didn’t see it in the earlier pics but you should try some shielded coax on both grids of v1. That should help with noise when adding more gain.
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Thank you guys for the input, I truly appreciate it. I do have some rg174 shielded coax here, so I will get to installing that and cleaning things up.
 
ibzprestige said:
Now looking for some little tweaks I can do to make it a little meaner, anyone have any suggestions?
Click to expand...
Awesome build! I recently got one of these and am going to be building it soon. Did you build it straight from the wiring diagram or make any changes?

The first thing I am going to do after I finish mine and play it for a while are these mods:
 
jchrisf said:
Awesome build! I recently got one of these and am going to be building it soon. Did you build it straight from the wiring diagram or make any changes?

The first thing I am going to do after I finish mine and play it for a while are these mods:
Click to expand...

I built it straight to the diagram. I also bought the components to do the JEL mod, I just wanted to have some fun with it before I went all out with the mod and make sure everything was working.

I did end up putting shielded cable on the input and gain pot to V1
 
ibzprestige said:
Bought the Mojotone British 50 Watt Master Volume kit, recieved it last Saturday and finished mid last week. I’ve done mods before but this was my first from scratch build, It was a lot of fun. I know some of the longer wires can be cleaned up, but there’s absolutely no noise when it’s on and turned up, so I’m in no rush with cleaning that up. Now looking for some little tweaks I can do to make it a little meaner, anyone have any suggestions? It is a 1978 JMP 2204 circuit.

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Wiring diagrams:

https://www.mojotone.com/Amp_Kits/British/British_50W_1978_2204_WD.pdf
Click to expand...
That is bad as fuck
 
You need to reflow all of your solder joints. Every one of them are cold soldered. The solder should flow into the joint and adhere. You need around 600 deg F.

Your solder iron also compromised one of your 470pF bypass caps across a 470k resistor. You should replace it as it will be prone to fail short.
 
glpg80 said:
You need to reflow all of your solder joints. Every one of them are cold soldered. The solder should flow into the joint and adhere. You need around 600 deg F.

Your solder iron also compromised one of your 470pF bypass caps across a 470k resistor. You should replace it as it will be prone to fail short.
Click to expand...
For someone getting ready to build one of these would you recommend any changes to the wiring diagram?
 
jchrisf said:
For someone getting ready to build one of these would you recommend any changes to the wiring diagram?
Click to expand...
I’m a big advocate for improving the stock ground schemes. Every pot should reference a starred ground attachment and not the chassis ground or potentiometer body like a stock amp does. As much as people hate on Larry and for good reason, his 5 star grounding scheme is what should be followed. You want to cut down on ground loops inside the chassis especially once you start going down the path of modding for high gain.

For a novice I suggest learning how to install coax correctly with only one side referencing ground and knowing where they should exist. Ask questions along the build.
 
glpg80 said:
You need to reflow all of your solder joints. Every one of them are cold soldered. The solder should flow into the joint and adhere. You need around 600 deg F.

Your solder iron also compromised one of your 470pF bypass caps across a 470k resistor. You should replace it as it will be prone to fail short.
Click to expand...
I did reflow everything after I got it all working and modded with the JEL spec. One question, I did that mod and installed coax on the input and gain pot, input is grounded to the ground lug of Jack, and gain pot is grounded to back the pot, one side only on each. I seem to now have a ground loop. And I didn’t before with the standard pvc wire. Any idea what I should look at to remedy it?
 
jchrisf said:
Awesome build! I recently got one of these and am going to be building it soon. Did you build it straight from the wiring diagram or make any changes?

The first thing I am going to do after I finish mine and play it for a while are these mods:
Click to expand...

I ended up doing this spec last night. It sounds really great. Definitely recommend trying it out.
 
Looks great man. It's a slippery slope though lol. You'll be planning your next build before this one is over.

I wouldn't worry about the wire lengths. If it's quiet, there is no need to mess with it
 
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